The transfer of liquid gasoline from one container to another container produces gasoline rich vapors which are normally displaced into the atmosphere as the container is filled. Such transfers occur daily at gasoline service stations, both during the transfer of gasoline from a bulk tank truck to an underground storage tank at the station and thence from the underground storage tank through its gasoline pumps to an automobile tank. Gasoline vapor losses at the service station principally arise from the underground storage tank, which is subjected to both breathing and displacement losses. Breathing losses are caused by alternate expansion and contraction of the tank contents due to day-night temperature differentials. Such temperature differentials are minimized by using buried tanks at gasoline storage stations. Displacement losses occur upon refilling a partially empty or empty storage tank which normally expels an equivalent volume of vapor into the atmosphere through the vent pipe of the storage tank.
If gasoline vapors contained in a storage tank above the liquid level of the tank are continuously replaced with fresh air, it is possible to vaporize a very large amount of liquid gasoline. In an automobile fuel tank, air replaces the volume of fuel consumed during driving. This tank air volume will be the sole source of air to replace liquid and vapors in the preceding storage tank from which liquid gasoline was drawn to fill the automobile tank; that is, the storage tank at the gas station. At the service station, vapors from the automobile fuel tank could ultimately be transferred through the storage tank to the emptied gasoline truck for return to the refinery ethyl.
In the event an automobile tank is refueled directly from a delivery truck tank which is normally vapor tight, the delivery tank will obtain its displacement vapor only from the vapor space of the automobile tank as the fuel is dispensed. Thus, from the automobile tank to the delivery tank, liquid is being exchanged for gasoline saturated vapor volume. If the two tanks are at the same temperature, then the exchange of volume will be on a one to one basis. But if the delivery tank temperature is higher, and colder tank displaced vapors come to equilibrium temperature, then all of the vapor from the automobile tank will not fit in expanded condition into the delivery tank and excess vapor will escape into the atmosphere as a vapor loss. If the delivery tank is cooler, then the vapors transferred to the delivery tank will contract and outside air must be sucked into the vent line of the delivery tank, or gas vaporized, or the tank pressure remains below atmosphere pressure to make up the difference in volume.
Prior to vapor controlled systems and when an automobile fuel tank had only one or two gallons of gasoline remaining, this small amount of gasoline was considered to be highly "weathered" because of engine heat, high agitation and vehicle tank ventilation. By "weathered" is meant that the gasoline has lost some of its more volatile components. Vapor space in the automobile tank is saturated with respect to volatile components and their mole fractions in the liquid and vapors. When the automobile tank is filled with fresh gasoline, more gasoline vapors are produced as gasoline is used reflecting the changed composition of the fresh gasoline. Volume of vapors discharged from the vehicle tank during refueling may be from 2% to 15% greater than the liquid volume of the gasoline dispensed. Various prior proposed sytems have been used to cope with this problem including vapor balanced transfer systems where liquid and vapor spaces are connected together between two containers in which liquid is to be transferred, absorption with lean oil, high pressure compression systems, adsorption of hydrocarbon vapors on activated charcoal, refrigeration of saturated vent gasses, compression and refrigeration of the vent gasses, and combustion devices to dispose of residual hydrocarbons in vented gasses.